1. Field of the Invention
The present invention relates to a process and an apparatus for refining silicon, particularly for enhancing removal of impurities such as P, Al and Ca by electron beam melting. The process and apparatus produce silicon having low concentrations of these impurities.
2. Description of the Related Art
In recent years, requirement for diversification in energy sources has caused solar power generation to be spotlighted. Generating apparatus of low cost has actively been researched and developed. Under such circumstances, silicon is likely to be most generally used as a raw material for solar batteries. Crystalline silicon has attracted the most serious attention as the material used for such a power supply.
It is recognized that high purity silicon having a purity of 99.9999% (6N) or more (hereinafter abbreviated as SOG-Si) is required for use as a raw material for solar batteries. Concentrations of impurities in the silicon have to be reduced to an order of parts per million or lower. A process has been proposed for producing high purity silicon from commercially available silicon metal (purity: 99.5%, hereinafter abbreviated as MG-Si), in which metallic impurities such as Al, Fe, Ti and the like are removed by directional solidification making use of small segregation coefficient. In this process C is deposited on the surface of the silicon by solidification, in the case of SiC, and is removed in the form of CO in the case of solid solution C; and B is removed by Ar plasma melting carried out while adding H.sub.2 O, CO.sub.2 or O.sub.2.
However, in the production process described above, the methods for removing the respective impurity elements are different from each other. The steps and facilities used are complicated. Moreover, a loss is caused in transfer from one step to a following step, and this lowers the silicon yield.
On the other hand, electron beam melting is usually used for melting high-melting metals such as titanium and molybdenum, and it is being researched for production of silicon used for a solar battery.
In Japanese Unexamined Patent Publication No. 61-232295, a copper-made vessel, cooled with water, is used in order to prevent contamination caused by vessel materials in electron beam melting of silicon metal. Japanese Unexamined Patent Publication No. 63-64909 discusses making a silicon sheet for a solar battery by combining a water-cooled copper hearth (having a small depth) with electron beam melting. Further, proposed in Japanese Unexamined Patent Publication No. 5-124809, is a solidification refining process in which a temperature gradient is provided in a vertical direction by melting only an upper part of a silicon metal held in a cast vessel, making use of local heating by an electron beam, and cooling the lower part of the silicon metal.
Also proposed is a method for removing P under reduced pressure by making use of its high vapor pressure. However, it takes a long time to remove P under reduced pressure. Recently, however, it is reported that P contained in silicon can be removed in a short time by electron beam melting [ISIJ International, vol. 32 (1982), No. 5, p. 635 to 642].
Further, an indicated advantage of electron beam melting is that Al and Ca as well as P can be removed together. However, in electron beam melting using conventional techniques, the removal limits for P, Al and Ca contained in silicon are about 3 ppmw, about 470 ppmw and about 150 ppmw, respectively. Since the concentrations of P, Al and Ca contained in silicon become almost fixed values in a melting time of 15 minutes or more, the contents of P, Al and Ca cannot be expected to be reduced any further. It is not reasonable to suspect that the electron beam melting method has been sufficiently investigated to achieve a purity required for SOG-Si, which is about 99.9999% Si or more.
We have independently proposed a so-called skull melting procedure, using water-cooled copper for a vessel (crucible, mold, hearth or the like) for holding the silicon, and that has made it possible to achieve a degree of purity as required for a silicon solar battery (Japanese Unexamined Patent Publication Nos. 7-309614 and 7-325827). High purity silicon is solidified at a location in contact with a water-cooled copper,vessel, and the silicon is melted in the inside of a solidified shell referred to as a skull. According to this method, contamination originating in impurities contained in the silicon adhered to the vessel can be prevented. Regrettably, however, this method has encountered the problem that about two-thirds of the energy held by the electron beam is taken up by the cooling water. Thus, the remaining energy contributing to melting is small, and very much reduces the heat efficiency of the treatment.